This invention relates to low molecular weight heterogeneous systems which harden to form polyurethanes when heated. These systems which may be fluid, paste like or readily fusible have a long shelf life and are based on polyhydroxyl compounds and 4,4'-diphenyl methane uret dione diisocyanates in dimeric and/or oligomeric form. Lead and/or tin catalysts may be employed. This invention also relates to a process for the production of such systems.
The use of isocyanates containing uret dione groups for the production of polyurethane plastics is known in the art. German Patent No. 1,014,740 for example, discloses that an NCO-prepolymer may be prepared from a polyester containing terminal hydroxyl groups and an excess of uret dione diisocyanate (for example dimeric tolylene diisocyanate). Subsequently, diols or triols, water, aromatic diamines, hydroxy polyesters or polyethers may be introduced into and mixed with the NCO-prepolymer. Crosslinking is accomplished by exposing the product to an elevated temperature.
It is also known that a storable polyurethane may be prepared from a relatively high molecular weight polyhydroxyl compound, glycol and stoichiometric quantities of diisocyanate and a uret dione diisocyanate. Polyurethanes containing OH-groups are first prepared from the polyhydroxyl compound, glycol and substoichiometric quantities of diisocyanates. Subsequently, the uret dione is introduced into and mixed with these polyurethanes containing OH-groups by any of the methods known and used by those skilled in the art of rubber chemistry. The mixture thus obtained is relatively stable in storage and forms a significantly, crosslinked, highly elastic material only after being heated to 100.degree.-180.degree. C. at which temperature the uret dione group reacts with the OH and/or urethane groups (See German Patent No. 968,566).
According to German Patent No. 952,940, a uret dione diisocyanate may be introduced into a polyurethane and mixed on rubber mixing rolls. The disclosed polyurethane contains terminal aromatic amino groups which polyurethane is produced from a relatively high molecular weight polyhydroxyl compound, aromatic diamines and substoichiometric quantities of aromatic diisocyanates. These polyurethane forming reactants are used in quantities such that free NCO-groups (based on the terminal amino groups) are present and an overall excess of isocyanate is obtained when the uret dione ring is opened by heating. By virtue of this excess of isocyanate, crosslinking accompanied by splitting of the uret dione ring occurs at temperatures above 100.degree. C.
It is also known that relatively high molecular weight polyhydroxyl compounds (such as polyesters or polyethers) and optionally difunctional chain extending agents (such as glycols) can be reacted in the melt with uret dione diisocyanates (dimeric 2,4-diisocyanatotoluene) or with mixtures of uret dione diisocyanates and other diisocyanates (such as 4,4'-diisocyanatodiphenylmethane) at temperatures below 100.degree. C. Such reactants are typically employed in quantities such that virtually all of the free NCO-groups of the diisocyanates are consumed by the reactive hydrogen atoms of the other reactants. The product is a storable, high molecular weight, substantially uncrosslinked polyurethane composition which may be converted into crosslinked polyurethanes by prolonged heating at temperatures above 100.degree. C. to react the uret dione group with, for example, urethane groups (see German Patent No. 1,153,900).
German Offenlegungsschrift No. 2,502,934 teaches that a polyisocyanate containing uret dione groups may be reacted with less than stoichiometric quantities (based on free NCO-groups) of a chain extending agent having a molecular weight in the range from 18 to 300 and containing from 2 to 3 NCO-reactive groups. The product of this reaction is converted by reaction with a blocking agent having a functionality of one into the corresponding compounds containing uret dione groups, and blocked isocyanate groups as well as urethane groups and/or reactive hydrogen atoms in the chain. The compounds thus obtained are reacted with a polyhydroxyl compound to produce polyurethane plastics by the isocyanate polyaddition process.
Addition compounds containing uret dione groups obtained from uret dione diisocyanates and NCO-reactive compounds are also described in German Publication No. 2,420,475, British Pat. No. 783,564, French Pat. No. 1,430,508, Belgian Patent No. 659,576, German Offenlegungs-schrift No. 2,221,170 and French Patent No. 1,471,610. In each of these patent specifications, the end product is hardened by crosslinking the compounds containing active hydrogen atoms and the incorporated uret dione ring and splitting the uret dione ring. Temperatures above 100.degree. C., generally above 140.degree. C. and, in some cases, up to 250.degree. C. are described as appropriate. Very long thermal processing times at high temperatures is necessary in some of these cases in order to complete hardening and to obtain the final properties required. The products containing uret dione groups made in accordance with these known procedures are generally based on dimeric diisocyanatotoluenes. These products are high molecular weight, thermoplastic intermediate products containing incorporated reactive groups (uret dione ring/H-active compound) even before they are completely hardened.
Only at a relatively high temperature (i.e. above 140.degree. C.) is the reaction by which the high molecular weight or crosslinked end products are formed takes place accompanied by softening or melting of the relatively high molecular weight urethane intermediates. At such high temperatures, the uret dione ring reacts to such an extent that the reactants initially fixed in the polyurethane matrix come into contact with one another through diffusion and are thus able to undergo polyaddition.
If, however, starting components such as those mentioned above (i.e. dimeric diisocyanatotoluene, polyesters, polyethers with or without low molecular weight chain extending agents) are mixed without preliminary modification (index below 100, based on free NCO-groups), there is an immediate increase in viscosity. Relatively high molecular weight, elastomeric intermediates and high molecular weight products (particularly where catalysts are present) form. These high molecular weight materials can only be processed--if at all--by elaborate techniques (as required e.g. in rubber technology) accompanied by crosslinking of the uret dione groups to form the crosslinked polyurethane. The relatively poor solubility and relatively high melting point (approximately 150.degree. C.) of these high molecular weight uret dione materials makes it difficult to work with them. If, however, dimeric diisocyanatotoluenes are used in combination with polyethers and/or polyesters and, optionally, low molecular weight chain extending agents (index approximately 100, based on free NCO-groups), it is possible to obtain reaction mixtures having a moderate or even relatively long processing time (pot life) at room temperature but these mixtures are no longer storage stable at room temperature under the conditions required for crosslinking (i.e. after addition of the necessary catalysts). Such mixtures are one-component systems characterized by an inadequate shelf life.
German Offenlegungsschrift No. 2,941,051 discloses a process for the production of fiber-reinforced moldings in which a polyurethane composition is produced from a high melting polyisocyanate (for example, dimeric 2,4-tolylene diisocyanate) and a composition containing reactive hydrogen atoms which has a functionality of from 2.5 to 3.5. The latter composition is produced from polyisocyanates, compounds containing from 2 to 8 OH-groups which have a molecular weight of from 300 to 10,000, a chain extending agent containing from 2 to 4 OH-groups, optionally a compound containing from 2 to 4 amino groups or water, and catalysts in the presence of a molecular sieve. However, the reactive components are storable for periods ranging from a few hours to at most a few days. The disclosed compositions are also disadvantageous in that only specially modified polyurethane compositions containing OH-groups may be used as reactants for the high melting diisocyanates.
German Offenlegungsschrift No. 2,921,162 describes fiber-containing molding compositions which are hardenable by heat. The disclosed compositions are made from a special prepolymer containing OH-groups (0.5 to 7% by weight of free OH-groups), from 5 to 20 wt. % urethane groups and optionally up to 2.5 wt. % urea groups, from 0.1 to 5 wt. % activators and, optionally from 1 to 5 wt. % molecular sieve which is hardened with a high melting polyisocyanate (melting point above 100.degree. C.) at a temperature above 90.degree. C. and under a pressure of from 20 to 400 bars. Dimeric tolylene diisocyanate is used as the high melting diisocyanate. Dimeric diphenyl methane diisocyanate may also be used. The molding compositions have a storage life of at least 10 days. However, the urethane-group-containing OH-prepolymer required can only be produced from a plurality of components in selected quantities and with selected functionalities.
German Offenlegungsschrift No. 2,842,805 also describes solvent-free one-component compositions which use a high melting diisocyanate (preferably dimeric tolylene diisocyanate), hydroxyl prepolymers pre-extended by urethane groups which contain dispersed hard urethane segments and, in addition, certain quantities of diamines. Branched polyethers having a functionality of from 2.5 to 3 must be used as the starter components. If the polyaddition reaction is carried out without first pre-extending the relatively high molecular weight polyol, the mixture containing dimeric tolylene diisocyanate is not storage stable if catalysts are used.
German Offenlegungsschrift No. 2,044,838 describes a process for the production of high molecular weight, substantially linear polyurethanes containing uret dione groups and free from NCO-groups. In this process, the relatively high molecular weight polyol, optionally certain chain extending agents and uret dione diisocyanates (for example dimeric tolylene diisocyanate or dimeric diphenyl methane diisocyanate) are reacted in the absence of catalysts in highly polar solvents (such as dimethyl formamide) either at room temperature or at slightly elevated temperatures. The NCO-groups of the dimeric diisocyanate are not storage stable in the highly polar solvent, because they react with the OH-groups in the polyols to form the linear polyurethane. In another step (for example spinning), the polyurethane containing uret dione groups is converted by reaction with aliphatic polyamines at room temperature into a high crosslinked product (for example filaments). In British Pat. No. 1,134,285, dimeric 4,4'-diisocyanato-diphenyl methane is disclosed as part of a one-component system. However, aqueous dispersions of dimeric 4,4'-diisocyanato-diphenyl methane in combination with H-acid compounds are claimed. These dispersions are hardened at 200.degree. C. and are said to be particularly useful for coating purposes (for forming thin films). However, the disclosed products are not defined MDI-uret diones, but instead relatively high molecular weight NCO-containing uret dione diisocyanates modified with urea groups (See Example 37 infra). The disclosed diisocyanates have extremely poor solubility and can only be reacted at temperatures above 200.degree. C. because the uret dione ring is split only under these temperature conditions.